This experiment set out to test how temperature shapes the composition of intertidal barnacle bed communities, asking the question: how do single vs. successive hot summers affect this same community?
Barnacle bed communities that are exposed to hotter temperatures during summer, even for a single year, will have lower diversity (species richness, Shannon-Weiner diversity, evenness, beta diversity) than those that are exposed to ambient/cooler conditions during the same period.
Warming will exert a persistent effect through time (i.e., warming in year one will stil affect communities in year two) mediated by changes in the cover of biogenic habitat.
Previously ‘cool’ communities, since they have more established, larger barnacle beds with a more diverse array of microhabitats and thermal refugia, will be less perturbed by warming than previously ‘warm’ communities that have less structurally complex biogenic habitat. That is, the effect of warming in year two will be contigent upon warming in year one.
This experiment was completed at Ruckle Provincial Park on the southeast-facing, semi-exposed sandstone shore of Salt Spring Island, located in British Columbia within the Salish Sea. Relative to the rest of the southern Gulf Islands, this site receives more substantial wave exposure and cooler, saltier water, being positioned more towards the Strait of Juan de Fuca and away from the Fraser River plume. Thus, the intertidal community at this site is substantially more diverse than neighbouring islands. However, like the rest of the Gulf Islands, this island’s intertidal zone is a “hot spot” in the region due to its mid-day summer low tides coupled with relatively clear, sunny weather during the summer.
The upper intertidal zone at this site is dominated by acorn barnacles (Balanus glandula and Chthamalus dalli), with sporadic beds of the perennial brown alga Fucus distichus and patches of the crustose phase of Mastocarpus sp.. Ephemeral algae can be found primarily in the winter when temperatures less stressful, namely the green ephemeral species Ulothrix sp. and Ulva sp., the red alga Pyropia sp., and the brown alga Petalonia fascia. Herbivores are relatively plentiful at this shore level, though some more thermally sensitive species migrate down shore with the onset of summer temperatures (Hesketh, personal observation). These include the littorine snails Littorina scutulata and L. sitkana and the limpets Lottia paradigitalis and congeners L. digitalis, L. pelta, and L. scutum.
Measurements in this experiment were made at the level of individual tiles deployed in the intertidal zone (Fig. 1). These tiles were manufactured as in previous studies employing the same passive warming method (Kordas et al. (2015)). In short, tiles consisted of two units made from high-density polyethylene (HDPE) plastic, a lower unit composed of thicker white HDPE (9.5 mm) anchoring the tile to the underlying bedrock, and an upper unit made of thinner HDPE (6.4 mm) that was either white (cool temperature treatment) or black (warm temperature treatment). A thin layer of Sea Goin’ poxy putty (Permalite Plastics) was spread in the central 6.9 x 6.9 cm area of the top unit to generate a settlement surface. To enhance the fine-scale heterogeneity of the surface, finely ground epsom salts were pressed into the putty as it dried, and dissolved with water after drying to leave behind fine pock marks. When tile colour was altered for a subset of tiles during the second year of the study, this was accomplished using heavy-duty tape, either white or black in colour (Gorilla Tape), with adhesion enhanced by the application of super glue.
Figure 1. Experimental tiles deployed at Ruckle Provincial Park, Salt Spring Island, pictured one year after their initial installation on shore. Recruitment and growth of algae and barnacles is evident in the central settlement area of each tile, while the outer black or white area of each tile serves to passively generate the warm and cool treatments used during this experiment, respectively.
The experiment followed a stratified random design, which went through several iterations as the original herbivore manipulation changed in response to methodological complications, and then again after the final question changed.
Figure 2. Stage 2 of experimental herbivore additions. Pictured here: L. digitalis (large, ribbed limpet) and L. paradigitalis (small limpets) added to a cool treatment tile in July 2019.